This invention relates to new diisocyanates containing urea groups which are liquid at room temperature or can be liquefied by heating to a temperature of up to 80.degree. C. These novel diisocyanates may be in the form of solutions in isocyanate prepolymers. The invention also is directed to a process for preparation of these new diisocyanates by the reaction of special diisocyanate starting materials and water and to their use as components for the production of polyurethanes.
It has long been known that the reaction of water with monoisocyanates leads to substituted ureas and the reaction of water with polyisocyanates leads to high molecular weight polyureas. Resinous polyureas containing isocyanate groups can be obtained according to U.S. Pat. No. 2,597,025 by using from 0.3 to 0.6 mol of H.sub.2 O per mol of aromatic diisocyanate in suitable solvents. It has also been disclosed that aromatic diisocyanates can undergo a selective reaction with water to produce low molecular weight diisocyanate ureas. U.S. Pat. Nos. 2,757,184; 2,757,185; and 3,906,019 describe the reaction of 2,4-diisocyanatotoluene with water under suitable reaction conditions to yield the corresponding bis-(3-isocyanatotoly)-urea. The analogous reaction of 2,6-diisocyanatotoluene to produce 1,3-bis-(3-isocyanatotolyl)-urea has been disclosed (U.S. Pat. Nos. 3,906,019 and 2,902,474). All these processes are carried out in solvents. The diisocyanates must be readily soluble in the solvents and the water added must be at least partially soluble. The solvent must not exert any polymerizing action on the isocyanate and must be free from isocyanate-reactive functional groups.
The main disadvantage of known diisocyanates containing urea groups is that when produced by known processes, they invariably are obtained as solids of widely varying particle sizes which are infusible or can be melted only at very high temperatures. Thus, processing of such solids for use as starting materials in the production of polyurethanes, for example, requires their conversion into finely divided form by elaborate grinding processes after they have been first isolated by filtration and vacuum treatment to remove solvents adhering to them. Furthermore, due to the high melting point and low solubility of these known diisocyanates containing urea groups, products obtained from reactions in which they are used are not homogeneous and often have poor mechanical properties. Due to the solid state and low solubility of urea diisocyanates known in the art, it is difficult to observe accurate equivalent ratios of isocyanate groups to isocyanate-reactive groups when processing them, since the ureadiisocyanate particles often react only on the surface so that the reaction products from an envelope enclosing unreacted urea diisocyanate.
However, diisocyanates which contain urea groups are valuable starting materials for the production of polyurethanes since the urea segments incorporated in the end products often improve the mechanical properties of the polyurethanes.
Objects of the present invention are to provide new urea diisocyanates for the production of polyurethanes having improved mechanical properties, to overcome the disadvantages of the urea diisocyanates known in the art, and to provide a process for the preparation of these urea diisocyanates which does not have the disadvantages of processes known in the art.
These objects surprisingly can be achieved by reacting certain sulfur-containing diisocyanates with water to produce the corresponding diisocyanates containing urea groups.